Most aero focused bikes will have dropped seatstays. As for ride comfort I don’t think it is important. I have 2 endurance road bikes and 1 has dropped stays (Giant Defy) and the other has full length stays (Canyon Endurace). Both have very similar feel. Seatpost flex and tyres appear to be the dominant factors for ride quality. I don’t think the seatstays do very much on modem carbon frames and are certainly not a consideration unless you have a strong aesthetic preference. Most people prefer the look of full length stays, but I don’t care either way.